Nicht‐Berry Pseudorotation beim Carbonylgruppenaustausch von Tetracarbonyl(η‐(Z)‐cycloalken)eisen‐Komplexen

Abstract

Exchange of Carbonyl Group Sites in Tetracarbonyl(η‐(Z)‐cycloalkene)iron Complexes via Non‐Berry PseudorotationThe tetracarbonyliron complexes of cyclobutene, cyclopentene, 4,4‐dimethyl‐cyclopentene, 2,5‐dihydrofurane, cyclohexen, (Z)‐cyclohepten, ‐octene, ‐nonene, and ‐decene were prepared by thermal or photochemical reaction of the corresponding olefins with nonacarbonyldiiron and pentacarbonyliron, respectively. The low‐temperature behaviour of the mostly new complexes which can be stored over a longer period only below 250 K and which exhibit four C,O‐stretching frequencies in the IR. spectra (cf. Table 2), indicative for a trigonal bipyramidal structure with the olefin ligand in an equatorial position, was studied in CCl2F2 by 13C‐NMR. spectroscopy between 200 and 115 K. In this temperature range all complexed olefin ligands with the exception of (Z)‐cyclooctene (cf. [11]) show an averaged Cs‐symmetry on the NMR. time scale. About 115 K the tetracarbonyliron group gives rise to three 13C‐signals in a ratio of 1:1:2 for the complexes of (Z)‐cycloheptene, (Z)‐cyclodecene and 2,5‐dihydrofurane (Cf. Table 3). This is an agreement with the fixed equatorial position of the non‐rotating olefin ligands. The complexes of cyclooctene and cyclononene give only two 13C‐signals in a ratio of 1:1 for the carbonyl groups. The temperature dependence of the signals indicates that in these cases the two axial carbonyl groups exhibit accidentally the same chemical shift. In all cases a complete line shape analysis of the 13C‐signals of the carbonyl groups could only be accomplished by using two exchange constants (cf. Tables 4 and 5 as well as Fig. 2–5 and Fig. 8). The same is true for the cyclobutene complex, but only one exchange constant could be determined (at 120 K: two 13C‐signals in a ratio of 1:3 with the beginning of a further coalescence). The cyclopentene and cyclohexene complexes showed only one 13C‐signal even at 115 K. The observed temperature‐dependent line shapes of the 13C‐signals can be interpreted in terms of a Non‐Berry pseudorotation mechanism involving a three site exchange with each of the two diastereotopic axial carbonyl groups and the two equatorial carbonyl groups (for activation parameters see Table 4). The differences in the activation parameters can be explained on steric grounds by assuming a transition state (cf. Fig. 8) similar to the C3b‐structure of tetracarbonyliron which lies about 27 kJmol−1 above its C2v‐structure (cf. [30]) comparable with the ground state of our complexes with weak dπ(Fe),pπ(Olefin) back bonding. The transition state model implies that the reorganization process involving the axial carbonyl group in exo‐position possesses the higher exchange barrier (cf. Fig. 8).